FIG. 1 illustrates a block diagram of a parallel transceiver module 2 currently used in optical communications, which has multiple transmit and receive channels. The transceiver module 2 includes a transmitter portion 3 a receiver portion 4. The transmitter and receiver portions 3 and 4 are controlled by a transceiver controller 6. The transmitter portion 3 comprises components for transmitting data in the form of amplitude modulated optical signals over multiple optical fibers (not shown). The transmitter portion includes a laser driver 11 and a plurality of laser diodes 12. The laser driver 11 outputs electrical signals to the laser diodes 12 to modulate them. When the laser diodes 12 are modulated, they output optical signals that have power levels corresponding to logic 1s and logic 0s. An optics system (not shown) of the transceiver module 2 focuses the optical signals produced by the laser diodes 12 into the ends of respective transmit optical fibers (not shown) held within a connector (not shown) that mates with the transceiver module.
Typically, a plurality of monitor photodiodes 14 monitor the output power levels of the respective laser diodes 12 and produce respective electrical feedback signals that are fed back to the transceiver controller 6, which processes them to obtain respective average output power levels for the respective laser diodes 12. The controller 6 outputs control signals to the laser driver 11 that cause the laser driver 11 to adjust the bias current signals output to the respective laser diodes 12 such that the average output power levels of the laser diodes are maintained at relatively constant levels.
The receiver portion 4 includes a plurality of receive photodiodes 21 that receive incoming optical signals output from the ends of respective receive optical fibers (not shown) held in the connector. The optics system (not shown) of the transceiver module 2 focuses the light output from the ends of the receive optical fibers onto the respective receive photodiodes 21. The receive photodiodes 21 convert the incoming optical signals into electrical analog signals. The transceiver controller 6 and/or other circuitry (not shown) of the transceiver module 2 processes the electrical signals to recover the data represented by the signals.
FIG. 2 illustrates a top view of a layout of a known parallel optical transmitter module of the type commonly used in the optical communications industry for simultaneously transmitting optical data signals over multiple optical transmit channels. The parallel optical transmitter module 22 shown in FIG. 2 has twelve optical transmit channels and zero optical receive channels. The module 2 includes a laser diode driver integrated circuit (IC) 23, a laser diode IC 26, a monitor photodiode IC 28, and a flex circuit 24. The driver IC 23 and the monitor photodiode IC 28 are mounted on opposite sides of the laser diode IC 26. This arrangement allows the driver IC 23 and the monitor photodiode IC 28 to be in close proximity to the laser diode IC 26, which has the following advantages. The wire bonds 31 that deliver the high-speed electrical signals from the driver IC 23 to the laser diode IC 26 should be relatively short so that the wire bonds contribute very little resistive loss, inductive coupling and electrical cross-talk. However, the driver IC 23 generates a relatively large amount of heat. Therefore, the wire bonds 31 should not be so short that heat generated by the driver IC 23 detrimentally affects the performance of the laser diode IC 26. The monitor photodiode IC 28 should be very close to the laser diode IC 26 in order to reduce optical cross-talk between adjacent photodiodes 29 and to allow the sizes of the photodiodes 29 to be kept relatively small, which reduces costs. Locating the ICs 23 and 28 on opposite sides of the laser diode IC 26 in the manner depicted in FIG. 2 generally enables these goals to be achieved.
Each of the ICs 23, 26 and 28 includes electrical contacts (not shown) that are electrically connected to electrical contacts (not shown) on the flex circuit 24. Only a portion of the flex circuit 24 is shown in FIG. 2. The flex circuit 24 extends downward in the module 22 in the direction toward the bottom of the sheet that contains FIG. 2, wraps around a surface (not shown) in the module 22, and then extends in a direction that is generally into the plane of the sheet containing FIG. 2. A portion of the flex circuit 24 located on the end that extends in this latter direction is electrically interconnected with electrical contacts on an upper surface of a ball grid array (BGA) (not shown).
One of the disadvantages of parallel optical transmitter modules of the type depicted in FIG. 2 is that the signal pathways that carry the high speed signals that drive the laser diodes of the laser diode IC 26 are extremely long. Each high speed signal pathway that delivers an electrical signal for driving one of the laser diodes 27 extends generally from the location where the corresponding electrical contact on the flex circuit 24 comes into contact with the corresponding electrical contact on the BGA to the location where the associated wire bond 31 comes into electrical contact with the laser diode IC 26. Thus, each of these high speed signal pathways generally has the following route: (1) through the flex circuit 24 from the corresponding electrical contact point on the BGA to the lower edge of the driver IC 23 (2) from the lower edge of the driver IC 23 to the upper edge of the driver IC 23, and (3) from the upper edge of the driver IC 23 to the lower edge of the laser diode IC 26. The high-speed signal pathways in the driver IC 23 that carry the signals for driving the laser diodes 27 are represented in FIG. 2 by numeral 37. The relatively low-speed control signals used by the driver IC 23 are typically carried in the regions of the IC 23 represented in FIG. 2 by numeral 38.
The extremely long lengths of the high speed signal pathways 37 often leads to undesired effects, such as, for example, cross-talk between adjacent signal path conductors, energy dissipation due to path losses associated with the electrical resistance of the pathway conductors, and relatively large signal path inductances caused by inductive coupling between adjacent signal pathway conductors. One or more of these undesirable effects can degrade signal integrity.
Another disadvantage of the parallel optical transmitter module 22 shown in FIG. 2 is that the pathways 37 through the driver IC 23 are extremely close together, which makes it impractical and/or extremely difficult to include elements in the pathways 37 other than the signal pathway conductive traces themselves and the transistors located along the pathways for providing the signals with the appropriate drive strength for driving the respective laser diodes 27. The distance between the pathways 37 is driven primarily by the distance between the centers of the cores of the optical fibers of the optical fiber ribbon cables (not shown) that are used with transmitter modules. This distance is typically referred to as “pitch”. The standard pitch for optical fiber ribbon cables is 250 micrometers (microns), which is also the standard pitch for the high-speed signal pathways of the transmitter modules of the type shown in FIG. 2.
One of the disadvantages associated with using this standard pitch for the high-speed signal pathways is that it can result in the aforementioned problems of cross-talk, resistive path loss and inductive coupling, which can degrade signal integrity. However, in order to increase the pitch between the high-speed signal pathways 37, the driver IC 23 would need to be significantly increased in width, which would also create a need to increase the width of the laser diode IC 26 and/or of the lengths of the wire bonds 31. If the width of the laser diode IC 26 is increased, the width of the monitor photodiode IC 28 would generally also need to be increased to maintain the optical relationship between each laser diode 27 and its respective monitor photodiode 29. Increasing the lengths of the wire bonds 31 may result in an increase in the risk of cross-talk, resistive path loss and inductive coupling problems. Increasing the width of the driver IC 23 and/or of the ICs 26 and 28 would generally result in an increase in the cost of the module 22.
A need exists for a parallel optical transmitter, receiver, or transceiver module in which the lengths of the high-speed signal pathways are significantly reduced without increasing the overall size and cost of the module.